Among all known materials, metals are very widely used because of their very advantageous properties: high levels of mechanical resistance and toughness, electrical conductivity, etc. Furthermore, their characteristic lustre is very much sought after for decorative applications. Their hardness is however inferior to that of ceramic materials, which may lead to significant wear in numerous common conditions of use, for example when these materials are used to make exterior parts used in watchmaking such as watch cases.
For this reason, one has for a long time sought to increase the hardness of metals. Conventional metallurgic methods (quenching, structural hardening) do not enable the high levels of hardness, greater than 1000 HV (Vickers hardness) required for certain applications, such as those for cutting tools or scratchproof decorative items, to be obtained.
For these applications, one has attempted further to increase the hardness of metals by the addition of very hard particles, by making two-phase composites formed of a metallic phase coating the hard particles, which frequently take up most of the volume of the material. A certain number of materials of this type have been developed and are today commonly used in the industry. They are generally called "hard metal" when the hard particles used are formed of tungsten carbide, and "cermet" when they are titanium carbonitride.
However, all these materials have in common the fact that they comprise a distinct metallic phase, which may limit their resistance to corrosion and their mechanical properties at high temperatures.
Further, this metallic phase always contains nickel or cobalt, all attempts to substitute other metals for these materials having failed until now. However, the use of this metallic phase with nickel and cobalt, in applications which lead to prolonged contact with the skin, such as for example watchmaking or jewellery applications, must be avoided for these elements are known to frequently cause allergies.
Finally, these materials are generally difficult to shape, which constitutes a significant limitation during manufacturing of exterior parts for watchmaking or parts for jewellery--which are often complex shapes--and makes their cost very high.